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THE BASIC OPEN-HEARTH PROCESS.                        199
dividuals of the original records, for it is found that low Si02 is accompanied by high FeO, and vice versa. This is shown by T'able XI-E, which is composed of the extreme cases of high and low percentages of SiO, and FeO, the individual heats which compose the groups in Table XI-D.
It would be wrong to suppose that an increase in Si02 has reduced the FeO by simple dilution, for a reduction in FeO from 20 per cent, to 10 per cent, would imply a permanent addition of Si02 equal to the entire volume of the slag, and this is absurd. The conclusion seems inevitable that Si02 and FeO replace one another in some way, and that one fulfils some function of the other. As FeO is basic and Si02 is acid, this function cannot be related to the basicity of the slag, and the only explanation which suggests itself is that' both confer fluidity and that there is an automatic regulation of this quality in accordance with the theory before elaborated.
SEC. XIj.—Determining chemical conditions.—Just as oxide of iron exists in slag in accordance with favorable conditions rather than with the initial character of the charge, so the content of phosphoric acid is governed by the chemical environment. The capacity of a cinder for phosphoric acid increases with the proportion of bases it contains, and lime is the most potent of these bases, but a certain fluidity is necessary, since a slag which is viscous does not seem to be as effective as one which is rendered fluid by oxide of manganese or iron. Thus, although lime is immeasurably superior to oxide of iron as a dephosphorizing agent, a slag containing a higher percentage of FeO is more efficient.*
One of the more important determinants of the capacity of slag for phosphorus is the phosphorus itself., The absorption of phosphoric acid is not a case of simple solution, like that of salt in water, but a union of acid and base, and each molecule of phosphoric acid which enters the slag decreases its capacity for more. It is impossible to prove this by ordinary averages, for the additions of lime are regulated by the demands of the silica rather than of the phosphorus, and it is a coincidence if the maximum content of phosphoric acid is present. Moreover, the determinmg conditions vary with each particular combination of the remaining elements, with the intensity of the reducing conditions, and the dura-
* The Open-Hearth Process.   Trans. A.I.M.E,, Vol. XXII, p. 446.